The spleen is located in the abdominal cavity, in the region of the left hypochondrium, at the level of the IX-XI ribs. The mass of an adult’s spleen is 192 g in men, and 153 g in women. It has the shape of a flattened and elongated hemisphere. Two surfaces are distinguished in it: the diaphragmatic and the visceral. On the visceral surface, there is a gate of the spleen, through which the splenic artery and nerves enter the organ, and a vein exits.
The spleen is covered on all sides by the peritoneum, which is firmly adhered to its fibrous capsule. Connective tissue beams (trabeculae) extend from the capsule into the organ. Between the trabeculae, the spleen parenchyma is called the pulp. Distinguish between white and red pulp. The white pulp is a typical lymphoid tissue of which periarterial lymphoid clutches and lymphoid nodules of the spleen or follicles are composed. Lymphoid nodules are round in shape and lie, as a rule, eccentrically with respect to the arteries. In lymphoid nodules with a multiplication center, there are dividing cells, young cells of the lymphoid series, macrophages.
Periarterial lymphoid muffs surround arterial vessels located in the spleen pulp. Lymphoid couplings are periarterial reticular tissue, densely filled with lymphocytes. There are also macrophages.
The red pulp occupies about 75-78% of the total mass of the spleen. In the loops of the reticular tissue of the red pulp are leukocytes, macrophages, erythrocytes, including decaying ones, and other cells. The splenic cords formed by these cells lie between the venous sinuses. In the red pulp there are also macrophage-lymphoid couplings (ellipsoids) that surround vessels of the sinusoidal capillary type and consist of densely packed reticular cells and fibers, macrophages and lymphocytes.
In newborns, the spleen can be oval, triangular or round in shape, it has a pronounced lobulation. The mass of the spleen in newborns is 8 g (by the age of five – 60 g), located at the level from VIII to X ribs (high position) or to XII ribs (low position). During the second childhood (8-12 years), the spleen takes on the same shape and position as in an adult.
The spleen is a kind of filter for the blood and the place where the immune response to antigens entering this organ through the bloodstream is carried out. In a healthy person, the spleen weighs 150 g and is enclosed in a thin, shiny bluish-gray capsule of connective tissue. On the section of the spleen, a red pulp is visible, mottled with gray spots – follicles.
The follicle includes an artery surrounded by an eccentrically located ring of T-lymphocytes – periarteriolar lymphoid clutches. At certain intervals, the couplings expand, forming lymphoid nodules, consisting mainly of B-lymphocytes, capable of forming germinal centers in response to antigenic stimulation, identical to those in the lymph nodes.
The red pulp of the spleen is crossed by numerous venous sinusoids with a thin wall, separated by splenic cords, or Billroth cords. The endothelial lining of the sinusoid has an intermittent structure, providing the exchange of blood cells between the sinusoids and the splenic cords.
The strands contain macrophages interconnected by long dendritic processes and create a physical and functional filter. Passing through the red pulp, blood reaches the splenic veins in two ways. In the first case (open circulation, or slow compartment), a certain amount of blood passes through the capillaries into the splenic cords, from which blood cells squeeze through the cracks in the discontinuous endothelial lining, entering the sinusoids.
In the second case (closed circulation, or fast compartment), blood quickly passes from the capillaries directly to the splenic veins. Almost the entire volume of blood during the day passes through the splenic cords, where it is “examined” by macrophages.
The spleen performs 4 functions that are directly related to the fight against diseases:
- phagocytosis of blood cells and corpuscular material . Erythrocytes undergo severe deformation during the transition from cords to sinusoids. Under conditions in which the elasticity of erythrocytes is reduced, they are captured in the cords and are rapidly phagocytosed by macrophages. Splenic macrophages are responsible for the pinpoint “cleaning” of erythrocytes (the process by which inclusions, such as Heinz’s body and Howell-Jolly’s little bodies, are excised), as well as for removing microorganisms, such as bacteria, and their fragments from the blood;
- production of antibodies . Dendritic cells in the periarterial lymphoid sleeves capture antigens and present them to T-lymphocytes. The interaction of T and B cells at the edges of the white pulp follicles leads to the formation of antibody secreting plasma cells, which are present mainly in the sinuses of the red pulp. Apparently, the spleen is an important source of antibodies to platelets and erythrocytes in immune thrombocytopenic purpura and immunohemolytic anemias;
- hematopoiesis . Under normal conditions, hematopoiesis in the spleen stops before birth, but it can be reactivated in severe anemia. The spleen is an important organ, since extramedullary hematopoiesis occurs in it in myeloproliferative diseases, for example, in chronic myeloid leukemia;
- depositing blood corpuscles . Normally, the spleen contains 30-40 ml of red blood cells, but this volume increases significantly with splenomegaly. In addition, the spleen usually contains 30-40% of the total mass of platelets in the body. In splenomegaly, up to 90% of the total platelet mass can be deposited in the interstitium of the red pulp, leading to thrombocytopenia. Likewise, leukocytes are deposited in the spleen, causing leukopenia.
The spleen, as the largest component of the mononuclear phagocytic system, is involved in all systemic inflammatory reactions, generalized hematopoietic disorders and many metabolic disorders. With each of these disorders, the spleen increases (splenomegaly), which is the main manifestation of the pathology of this organ. The spleen is rarely the primary site of disease.
The main clinical manifestation of insufficient spleen function after splenectomy or autoinfarction (with sickle cell anemia) is an increased susceptibility to sepsis caused by capsular bacteria such as pneumococcus, meningococcus, and H. influenzae. Loss of filtering function and antibody production increases the risk of sepsis, which can lead to death. All individuals with no spleen should be vaccinated against the appropriate pathogens to reduce the risk of this formidable complication.